Growing up in 1970s Suffolk in a crumbling giant of a house with wild, tangled gardens, the celebrated jeweller was left to wreak havoc by invention. Without visible parental influence but with sisters to love him and brothers to fight for him, he made nature into his world. Creation became a compulsion, whether it was go-karts and guns, cross-bows and booby-traps, boats, bikes or scooters. And then it was jewellery. He talks to Georgina Godwin.